Accept a function to strip like rstrip and lstrip

`strip` is documented to accept a function argument but it does not.
`rstrip` and `lstrip` do, so it seems this was simply an oversight.

See #31195.

NEWS.md

@@ -44,6 +44,7 @@ Standard library changes
 * `startswith` and `endswith` now accept a `Regex` for the second argument ([#29790]).
 * `retry` supports arbitrary callable objects ([#30382]).
 * A no-argument construct to `Ptr{T}` has been added which constructs a null pointer ([#30919])
+* `strip` now accepts a function argument in the same manner as `lstrip` and `rstrip` ([#31211])
 
 #### LinearAlgebra
 

base/strings/util.jl

@@ -195,15 +195,20 @@ rstrip(s::AbstractString) = rstrip(isspace, s)
 rstrip(s::AbstractString, chars::Chars) = rstrip(in(chars), s)
 
 """
-    strip(str::AbstractString, [chars])
+    strip([pred=isspace,] str::AbstractString)
+    strip(str::AbstractString, chars)
 
-Remove leading and trailing characters from `str`.
+Remove leading and trailing characters from `str`, either those specified by `chars` or
+those for which the function `pred` returns `true`.
 
 The default behaviour is to remove leading whitespace and delimiters: see
 [`isspace`](@ref) for precise details.
 
-The optional `chars` argument specifies which characters to remove: it can be a single character,
-vector or set of characters, or a predicate function.
+The optional `chars` argument specifies which characters to remove: it can be a single
+character, vector or set of characters.
+
+!!! compat "Julia 1.2"
+    The method which accepts a predicate function requires Julia 1.2 or later.
 
 # Examples
 ```jldoctest
@@ -212,7 +217,8 @@ julia> strip("{3, 5}\\n", ['{', '}', '\\n'])
 ```
 """
 strip(s::AbstractString) = lstrip(rstrip(s))
-strip(s::AbstractString, chars) = lstrip(rstrip(s, chars), chars)
+strip(s::AbstractString, chars::Chars) = lstrip(rstrip(s, chars), chars)
+strip(f, s::AbstractString) = lstrip(f, rstrip(f, s))
 
 ## string padding functions ##
 

test/strings/util.jl

@@ -53,6 +53,7 @@ end
     @test strip(" \u2009 hi \u2009 ") == "hi"
     @test strip("foobarfoo", ['f','o']) == "bar"
     @test strip("foobarfoo", ('f','o')) == "bar"
+    @test strip(ispunct, "¡Hola!") == "Hola"
 
     for s in ("", " ", " abc", "abc ", "  abc  "),
         f in (lstrip, rstrip, strip)